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My 6S is using a Straight Talk SIM and it still doesn't have VoLTE. It's 'enabled' in settings, but call quality is low enough to recognize it as NOT being VoLTE. Same with my Galaxy S5 before that. I've noticed that '1x' also displays as the data type at work, as well as during calls.

Doesn't work if your SIM was never provisioned for VoLTE.
I agree that the SIM must be provisioned. I’ve gotten several new sims over the years when I upgraded my iPhones.

AT&T spells out the requirements below

 
My Straight Talk SIM is a Verizon-based one. AT&T has a serious issue especially on pre-paid where you get tons of spam texts from email addresses that cannot be blocked (since it's from an email not a phone #) and it got bad enough to stop using AT&T, it was an issue on three lines based on them, and my mom's as well. She lives with it, but I cannot.
 
Might be my area then. It's pretty bad for anyone using AT&T. My coworker's mother uses AT&T on a iPhone X and has the same issue. She ignores all those messages. The spam texts come from emails (show an email instead of a phone number) and can be as many as five per day. This thread talks about it, and it's not resolved:



I've only seen this problem on AT&T and on any MVNO using AT&T's towers.

This thread here talks about some type of article you can tell an AT&T rep to stop emails from being able to send texts to your specific number, but unfortunately for me, none of the Indians at Straight Talk's customer care can leave their script enough to fix it. So it was either live with it or switch to a different SIM for a carrier unaffected by it:

 
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